The present invention concerns improvements in yarn feeders for textile machines and, in particular, in weft feeders for weaving looms, of the type in which the rotary drum on which the yarn is wound to form a reserve is held stationary and the turns of the yarn reserve are wound thereon by a rotary winding arm and are moved forward, mutually spaced, by suitable means.
As is known, in these machines, the yarn wound on the drum--properly positioned and with a practically constant tension--forms a yarn reserve for the textile machine to be fed. Particularly in the case of weaving looms, it forms a weft reserve for the loom, which can draw it so as to insert it into the warp quite independently from the ways and means adopted for its winding on the drum.
This hence requires an efficient and continuous control of the amount of yarn reserve on the drum, which has to be automatically kept substantially constant, with the least possible variations of the rotation speed of the rotary winding arm, that is, of the weft feeder motor.
For the control of the yarn reserve with mutually spaced turns, there are already known to be (for example from the DE-A-2934024) electromechanical means positioned externally to the drum and essentially consisting of one or more feeling rods, the position of which depends on the presence or the absence of yarn reserve turns in the drum area under control.
The position of the feeling rods causes in turn the operation of a transducer which generates an electric signal indicating the presence or the absence of yarn reserve.
It is also already known to use two electromechanical feelers, signaling a minimum and a maximum yarn reserve, with which there is associated an electronic circuit which processes the two signals and controls the winding speed of the reserve, so as to keep it as far as possible within the minimum and maximum limits.
There are also known to be (for example from the EP-A2-171516) other electromechanical means of the same type, the position of which again depends on the presence or the absence of yarn reserve, but which are positioned inside the drum and not externally thereto. These means are associated with a transducer, external to the drum, so as to be able to control, as in the previous cases, the winding of the yarn turns.
All such means detect the presence of weft yarn by contacting the yarn turns, thereby failing to satisfy, at least partly, the primary object of each weft feeder which is to put at the disposal of the loom, having to draw them, a plurality of reserve turns evenly arranged and wound with the slightest possible tension.
In fact, when working with fine and thus particularly delicate yarns, the mere contact with any one of the aforecited electromechanical devices may be sufficient to cause, from time to time, irregularities in the positioning of the turns, with consequent breakage when the reserve is being drawn by the loom.
Furthermore, in order to enable the turns to resist the pressure of the mechanical feeler, it may be necessary to brake the incoming yarn so as to obtain a more highly tensioned winding, thereby stressing the yarn even further and possibly breaking it.
On the other hand, if the pressure of the mechanical feeler on the yarn turns is reduced below a certain value the problem arises of the inevitable presence of dust produced by the yarn itself, which will soon strongly limit or even prevent the freedom of movement of said feeler.
To avoid all these problems invariably caused by the use of electromechanical means to control the yarn reserve, which means detect the presence of the turns by feeling them, use can be made--as is already done in practice--of photoelectric devices, which detect the presence of the turns by causing the same to intercept suitable light beams.
Such devices, when operating on weft feeders with mutually spaced turns of yarn reserve, may however easily supply wrong information in that, if they are not very sensitive, they are not adapted to distinguish between the presence and the absence of single yarn turns, whereas, if they are sensitive enough to detect single yarns, they are not in a position to distinguish between the presence of a turn of the reserve and the passage of a yarn leaving the weft feeder, drawn by the loom.
This drawback can be eliminated by making sure that the spaces between the turns of the reserve are not too wide, since in this case, while the reserve interferes with the light beams from the photoelectric means as a surface, the yarn leaving the weft feeder drawn by the loom interferes therewith as a line. It is thus easy to obtain different signals in the two cases, with a suitable sensitivity of the photoelectric means, and to hence control the motor of the weft feeder only with those signals generated by the presence of the reserve.
Solutions of this type are already known in the art.
For example, according to the EP-A-164032, the yarn turns are drawn close in the area of control on the drum, practically contacting each other in the case of working with thick yarns.
This solution limits however the possibility to select a space (pitch) of any width between the reserve turns, and to keep it unvaried along the whole winding drum, as can besides be required in the case of working "flat yarns", in order to prevent them from overlapping, or fluffy yarns, in order to prevent the fluff of two adjacent turns from interweaving, thereby causing breakage of the yarn while it is being drawn by the loom.
Furthermore, with this system, the even considerable differences in the dimensions of the yarns are adapted to create further elements of doubt as far as detecting the reserve by the photoelectric means. This occurs in particular with thick yarns.
This solution requires moreover a very efficient self-controlling system of the photoelectric devices, as one has to take into account the dust deposits which rapidly form on the lenses of the photoelectric cells. This creates difficulties of construction and problems which cannot always be satisfactorily solved in order to combine efficiency and economical advantages.